Diaphragm metering pumps frequently are used to dose, inject or transfer hazardous or corrosive fluids into a process stream. In the event of a failure in the diaphragm seal area, these fluids, or pump hydraulic oil, may contaminate the immediate environment. Conversely, in the case of a diaphragm failure due to fatigue in the diaphragm working area (as opposed to the radially outer seal area), a process may be ruined due to hydraulic oil being injected into the process stream. Alternatively, a corrosive fluid may be drawn into the metering pump, thereby causing severe corrosion damage. One partial solution to this problem is to provide a double or triple diaphragm construction. In one such pump, spaces between the diaphragms are filled with an inert fluid which transmits the fluid pressure from the working fluid to the process fluid. In a second type of pump, the diaphragms are positioned closely together and a thin film of lubricant may be provided in the space between the diaphragms. While these constructions reduces the incidence of diaphragm failure, leaks still can occur and hence the ability to detect a diaphragm failure due to either seal area breakdown or working area fatigue failure is considered of prime importance in many industries. In the fluid-filled intermediate chamber type of pump described above, this may be accomplished by sensing a change in conductivity of the intermediate fluid. In the second type of pump, leakage detection is achieved by sensing leakage of working or process fluid into a leakage port. An example of the latter is shown in FIGS. 1 and 2, which illustrate a prior art double diaphragm metering pump having first and second diaphragms 10, 12 separated by a fluid-filled space 14. A hollow annular ring 16 is located at a seal region 18 of the diaphragms 10, 12 and the hollow ring 16 and the diaphragms 10, 12 are clamped between a diaphragm head 20 and a member 22 defining a displacement chamber (not shown). A recess 24 in the hollow member 16 is in fluid communication with a hollow tube 26, which in turn is connected in fluid communication with a pressure gauge 28 (FIG. 1).
As can be seen by an inspection of FIG. 2, there is a roughly triangular space 30 located just radially inward from the radially innermost part of the hollow member 16. This space 30 results from the clamping of the diaphragms 10, 12 by first and second shim pads 32, 34 against a tip 36 of the hollow member 16. The space 30 can trap air which can interfere with the ability of the diaphragms to operate as a unit. Also, high stresses due to clamping of the diaphragms just below the space 30 can lead to fatigue failure.
FIG. 3 illustrates a further prior art pump 40 wherein first and second diaphragms 42, 44 are separated at a radially outermost portion by a machined ring 46. The diaphragms 42, 44 and the machined ring 46 are clamped between members 48, 50 and 52. An O-ring 54 provides sealing between the members 48, 50. A duct 53 is placed in fluid communication with a pressure gauge or pressure switch (not shown) to indicate when a leak has occurred which causes either working fluid or process fluid under pressure to enter the space between the diaphragms 42, 44. While the machined ring 46 minimizes the volume of trapped air between the diaphragms 42, 44, it has been found that this design can only be operated at pressures below the rated pressure of the pump 40 owing to the diaphragm design.